1. Field of the Invention
The invention relates to a process for treating fine ore, wherein the fine ore is dried by aid of a hot drying gas flowing around the ore particles of the fine ore, the drying gas, after having flown around the ore particles, is purified while releasing entrained dust ore particles and the dust ore particles are collected and admixed to the dried fine ore, as well as an arrangement for carrying out the process.
2. Description of the Related Art
A process of this type is known from U.S. Pat. No. 5,082,251. There, drying of the fine ore is effected in a rotating drum type drier.
The further processing of fine ore, such as the direct reduction of fine ore, involves difficulties if dust ore particles are contained in the fine ore in excessive amounts, in particular, dust ore particles having dimensions smaller than 150 .mu.m. An excessive portion of such dust ore particles in the fine ore is likely to give rise to serious process disturbances in further processing caused, for instance, by obstructed ducts, cyclones or by the formation of deposits in reactor vessels. Such disturbances also may arise if the portion of dust ore particles is acceptable as a whole, yet the distribution of the dust ore particles within the fine ore strongly varies. Consequently, maximum contents of dust ore particles may occur, lying considerably above the dust ore particle portion to be coped with without any disturbances; such maximum content portions may amount up to 50%.
From EP-A-0 149865 it is known to subject sulfidic ores to whirl bed drying. With this known process, the total amount of solids is supplied to gas purification together with the exit gas from whirl bed drying through a duct, the solids separated in purification being admixed to the previously separated solids without considering their grain dimensions. Thus, no wind-screening is provided there.
From CH-B-318.904 it is known to finely pulverize ore oxides and to subject the incurring powdery stock to wind-screening by using reducing gas for wind-screening. The coarser iron powder retained in wind-screening is returned to the pulverization procedure such that uniformly fine powder ore particles exclusively will be available to the reduction of the ore.
DE-A-1 458 755 relates to a process for controlling the grain size distribution and the fine grain concentration in whirl-layer reaction zones. There, a fine grain concentration ranging between 5 and 60% by weight is adjusted in the whirl layer by withdrawing fine grains of a predetermined grain size range from the whirl layer and returning them back into the whirl layer in circulation in given amounts. What is important there is to counteract the enrichment of the amount of fine grains contained in the whirl-layer reaction zone due to the formation of new fine grains caused by abrasion in that zone. Hence, that process does not deal with the treatment of fine ore, but with a process taking place during its reduction.